Unprofessional SERVICE from TMOBILE

Recently moved from Verizon to T-Mobile. We got Samsung Z flip phone and it stopped working within 3 months. We went to a store to report this and they acknowledged and ordered a replacement phone. After the new phone came to the store, we went to pick up. The guys at the store said sorry your current phone not working so can't replace with the new phone. How stupid it was... Two weeks earlier they inspected our phone and then they ordered a new phone for us, but when we went to pick up all the hassle created and waster our time and effort. 

can T-Mobile stand on their promise or at least stick to what has been agreed rather than changing at their own will.